Flexography is today one of the most important printing techniques. It is used for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Coarse surfaces and stretch films can only be printed economically with flexography, making it indeed very appropriate for packaging material printing.
Today flexographic printing masters are prepared by both analogue and digital imaging techniques. Analogue imaging typically uses a film mask through which a flexographic printing precursor is exposed. Digital imaging techniques include:                Direct laser engraving as disclosed in e.g. EP-As 1710093 and 1936438;        UV exposure through a LAMS mask wherein LAMS stands for Laser Ablative Mask System as disclosed in e.g. EP-A 1170121;        Direct UV or violet exposure by laser or LED as disclosed in e.g. U.S. Pat. No. 6,806,018; and        Inkjet printing as disclosed in e.g. EP-As 1428666 and 1637322.        
The major advantage of an inkjet method for preparing a flexographic printing master is an improved sustainability due to the absence of any processing steps and the consumption of no more material as necessary to form a suitable relief image (i.e. removal of material in the non printing areas is no longer required).
EP-A 641648 discloses a method of making a photopolymer relief-type printing plate wherein a positive or negative image is formed on a substrate by inkjet printing with a photopolymeric ink, optionally preheated to a temperature between 30 and 260° C. and subjecting the resulting printed substrate to UV radiation, thereby curing the ink composition forming the image.
U.S. Pat. No. 6,520,084 discloses a method of preparing flexographic printing masters by inkjet wherein a removable filler material is used to support the relief image being printed and wherein the relief image is grown in inverted orientation on a substrate. Disadvantages of this method are the removal of the filler material and the release of the relief image from the substrate.
EP-A 1428666 discloses a method of making a flexographic printing master by means of jetting subsequent layers of a curable fluid on a flexographic support. Before jetting the following layer, the previous layer is immobilized by a curing step.
In U.S. Pat. No. 7,036,430 a flexographic printing master is prepared by inkjet wherein each layer of ink is first jetted and partially cured on a blanket whereupon each such layer is then transferred to a substrate having an elastomeric floor, thereby building up the relief image layer by layer. A similar method is disclosed in EP-A 1449648 wherein a lithographic printing plate is used to transfer such layers of ink to a substrate.
US20080053326 discloses a method of making a flexographic printing master by inkjet wherein successive layers of a polymer are applied to a specific optimized substrate. In US20090197013, also disclosing an inkjet method of making a flexographic printing master, curing means are provided to additionally cure, for example, the side surfaces of the image relief being formed. In EP-A 2223803 a UV curable hot melt ink is used. Each of the deposited layers of ink is gelled before the deposition of a subsequent layer on the deposited layer. After the printing master with sufficient thickness is formed, the ink is cured.
EP-As 1637926 and 1637322 disclose a specific curable jettable fluid for making flexographic printing masters comprising a photo-initiator, a monofunctional monomer, a polyfunctional monomer or oligomer and at least 5 wt. of a plasticizer. The presence of the plasticizer is necessary to obtain a flexographic printing master having the necessary flexibility. Also in EP-A 2033778, the curable jettable fluid for making a relief image by inkjet on a sleeve body contains a plasticizer.
A flexographic printing master formed on a support by an inkjet method typically comprises an elastomeric floor, an optional mesa relief and an image relief as disclosed in EP-A 2199082.
To realize a high resolution with such an inkjet method, particularly for printing the image relief which determines the finally printed image, it is advantageous to use a printhead with small nozzle diameters, for example producing 3 pl fluid droplets. However, such a small nozzle diameter requires low viscosity inks.
A jettable fluid comprising a plasticizer as prescribed in EP-As 1637926, 1637322, 2199082 and 2199081 has two disadvantages. Firstly, the plasticizers may migrate towards the surface of the image relief as a function of time or may be extracted out of the relief image by the printing ink during the flexographic printing process. This may result in a change of the physical properties of the image relief and as a result thereof of the printing properties of the image relief. Secondly, the presence of plasticizers in the ink often results in a too high viscosity.